Like pretty much anything that Melissa McCarthy does these days, your enjoyment of “Tammy” will depend entirely on how you feel about the actress as a performer. Those who can’t get enough of watching her play the same sloppy and obnoxious character over and over again will probably think that it’s the funniest comedy of the year. But for those who were already sick of her tedious, one-trick pony act after “Bridesmaids” launched the actress into superstardom, sitting through McCarthy’s latest movie is about as pleasant as a punch to the face. “Tammy” is so groan-inducingly dumb that it rivals some of Adam Sandler’s worst comedies, placing so much faith in its leading lady’s raucous, over-the-top antics that it doesn’t even consider it might not be funny.

McCarthy stars as the title character, a slovenly loser who wrecks her beat-up car, gets fired from her job at the local fast food joint, and discovers that her husband (Nat Faxon) has been cheating with their neighbor (Toni Collette), all within the same day. Desperate to get out of town for a while, she agrees to let her alcoholic grandmother, Pearl (Susan Sarandon) – who has the two things that Tammy needs most: a working car and some cash – tag along with her on a road trip to Niagara Falls. But when they end up driving the wrong way, the two women decide to make the most of the mishap in an attempt to patch up their troubled past.

McCarthy clearly didn’t get the memo that her 15 minutes of fame are up, because the actress (whose last few films suggest that she’s aspiring to be a female Chris Farley) keeps plugging away with stupid movie after stupid movie. And aside from the fact that studios continue to bankroll these so-called comedies, the worst part is that every character she plays is a variation of the same role, although she does switch things up slightly with “Tammy” by adding “idiot” to her list of personality traits, as if that somehow endears her more to the audience. Of course, it doesn’t help that the one person capable of reining McCarthy in (director Ben Falcone) happens to be her husband, with whom she co-wrote the script, making the whole thing feel like one big vanity project.

The problem, however, is that none of it is particularly funny. McCarthy earns exactly one laugh the entire film, while Sarandon (wearing an old lady wig to hide the fact that she’s only 24 years older than her co-star) is saddled with lame jokes about her age and drinking problem. Even the movie’s cavalcade of guest stars are wasted in throwaway roles, with some barely given a few lines of dialogue because McCarthy is too busy yammering every chance she gets. It’s bad enough that she never shuts up, but Falcone wants the audience to sympathize with Tammy, even though she’s largely to blame for much of what happens to her over the course of the film. She’s not as mean-spirited as Diana from “Identity Thief,” nor as vulgar as Shannon from “The Heat,” but she’s not someone you’d necessarily want to be friends with either.

So when the movie suddenly tries to manufacture a romance between Tammy and Mark Duplass’ nice-guy farmer Bobby in the final act, it doesn’t just feel unearned, but completely unrealistic considering Bobby had zero interest in her only a few days prior when they were first introduced. That meeting takes place shortly after an exceedingly confident Tammy brags to her grandmother about how men are attracted to her like “flies on shit,” and in that instant, McCarthy provides the perfect ammunition to describe her recent career, because that’s what this movie is – a big, steaming pile of shit.